Northern Prairie Wildlife Research Center
Closed containers such as vials and plastic bags will keep the foliage fresh over longer periods, but they tend to have high humidities that promote the growth of molds. Sprigs of foliage in miniature vases or water picks can be utilized to good measure. In this way it is possible to keep the foliage fresh for days even in open containers. "Sleaving" is a low-maintenance rearing method whereby eggs or larvae are placed in fine meshed bags and secured over a shoot or limb with generous foliage. If enough leaves are sleaved, larvae will complete their development with minimal disturbance. This technique frequently produces a high yield of full-sized adults.
Many species pupate in litter or below ground, so we suggest that you provide a 1-inch layer of slightly moistened sphagnum or peat moss (processed sphagnum) in each rearing container. Sphagnum tends to discourage the growth of molds and releases moisture slowlythis can be especially helpful to those species that spend many months in the prepupal or pupal stage. For example, many of the moths that specialize on spring foliage as caterpillars may spend 10 months on or below the ground. A few dagger and forester moths require a denser pupation medium, as they normally fashion their pupal chambers in rotten wood. A small chunk of pulpy or punky wood added to the rearing container can greatly increase rearing success with such species.
An extended period of near-freezing temperatures is usually necessary to ensure emergence of species overwintering as larvae or pupae. We have had success holding larvae in a refrigerator at 2 to 5 °C (36 to 41 °F) for several months; a garage or woodshed that does not drop below -4 °C (25 °F) will also serve well. Addition of moisture (simulated rain) will help keep livestock from desiccating over the winter months. Should you decide to bury your collections or cover them with leaves, take precautions to exclude mice and other insectivorous mammals active over the winter months. Many helpful suggestions for collecting and rearing caterpillars can be found in Covell (1984) and Villiard (1969).